1. Field of the Invention
The present invention relates to optical transmission, and more particularly to an Ethernet passive optical network for providing a subscriber with a high speed and large capacity data service and a real time digital broadcast/video service.
2. Description of the Related Art
A data transmission rate of above 100 Mb/s is required for high speed, real time, service of a combination of large capacity data and digital broadcast/video. Since, however, currently used xDSLs or cable modems have a data transmission rate of 50 Mb/s at maximum, xDSLs and cable modems cannot meet this challenge. Pursuant to studies and research, an optical access network has been suggested as a method for meeting such a requirement. In particular, a passive optical network (PON) has been proposed as an economical optical access network.
Such a PON may be an ATM-PON based on the ATM (asynchronous transfer mode) protocol, a WDM-PON based on a WDM (wave division multiplexing), or an Ethernet PON (E-PON) based on Ethernet. A fiber to the home (FTTH) version of an Ethernet PON structure has been suggested and developed for high speed optical transmission to a home.
Ethernet PONs have generally been developed to fundamentally process communication data. The Ethernet PON employs a wavelength of 1550 nanometers (nm) to transmit data from an optical line termination (OLT) to a plurality of optical network terminations (ONTs) This entails transferring gigabit Ethernet signals at a data rate of 1.25 Gb/s and at a wavelength of 1310 nm. The Ethernet PON has required broadcasting signals, however, as demand for broadcasting services using an optical access network has increased.
To this end, and referring to FIG. 1, suggestion has been made for an overlay broadcasting processing method for transmitting broadcasting signals to a plurality of ONTs by using broadcasting signal wavelengths, which are different from communication data wavelengths. FIG. 1 portrays a conventional Ethernet PON for broadcasting/telecommunication convergence, which includes, positioned between a user and a service node, an optical line terminal (OLT) 100. The latter receives and electro-optically converts a broadcasting signal and a communication signal delivered from a broadcasting vendor and a communication vendor, respectively, and sends the broadcasting signal and the communication signal as a combined optical signal. The conventional Ethernet PON also includes, at the users' side, optical network terminals (ONTs) 200-1 to 200-N for delivering to users information received from the OLT 100. A passive optical splitter 118 and an optical cable connect the OLT 100 to the ONTs (200-1 to 200-N).
The OLT 100 converts optical broadcasting signals delivered through a broadcasting network into optical signals for downstream transmission by means of an opto-electrical converter 115, followed by an electro-optical converter 116. The converted optical signals are amplified by an erbium doped fiber amplifier (EDFA) 117, and the amplified signals are transmitted downstream. The OLT 100 also receives communication data from an internet protocol (IP) network through an IP router 111 and converts the communication data into optical signals by means of an E-PON OLT function processing part 112 so as to transmit the optical signals by means of a transmitter 113. The OLT 100, on the other hand, receives data from the ONTs 200-1 to 200-N and transmits the received data through the IP router 111 to the IP network.
The ONTs 200-1 to 200-N consequently receive, by means of broadcasting receivers 119-1 to 119-N, the broadcasting signals and deliver the broadcasting signals to users through broadcasting set-top boxes 122-1 to 122-N. As to the communication data, the ONTs 200-1 to 200-N receive it by means of receivers 120-1 to 120-N and deliver it to users through E-PON ONT function processing parts 123-1 to 123-N. In addition, the ONTs 200-1 to 200-N receive upstream communication data from a user by means of the E-PON ONT function processing parts 123-1 to 123-N and forward the received data sent by means of burst mode transmitters 121-1 to 121-N.
The EDFA 117 is expensive, and is required to deliver analog broadcasting signals to the ONTs 200-1 to 200-N. Even if only digital broadcasting is processed an expensive EDFA is still required if the number of digital broadcasting channels increases.
Since all broadcasting channels are transferred to the ONTs 200-1 to 200-N, the ONTs 200-1 to 200-N require expensive optical receivers featuring great receiving-sensitivity, superior noise-characteristics, etc., in order to receive broadcasting signals transferred from the OLT 100. Additionally, an optical transmitter required for the OLT. Moreover, although subscribers may require high definition and real-time digital video services as well as digital broadcasting services in the future, it is difficult for the conventional Ethernet PON to process high definition and real-time digital video services.
In order to solve the above described problems, an Ethernet PON has been proposed in which broadcast/video channels are time-division multiplexed together with GbE (Gigabit Ethernet) communication data and are transferred to an ONT. Accordingly, it is unnecessary to employ an EDFA and it is unnecessary for an OLT and an ONT to additionally have an optical transmitter and an optical receiver, respectively. In addition, the Ethernet PON ensures quality of service (QoS) for high definition digital video, to be required by subscribers in the future, as well as for digital broadcasting. However, in the proposed Ethernet PON, a broadcast/video time-slot is specifically assigned to every subscriber and is used only for transmitting broadcast/video. Also, when a subscriber does not look at and listen to broadcast/video, a time-slot assigned to the subscriber is not used. For example, if the E-PON has a 1×16 structure and broadcast/video data, e.g., an MPEG transport stream (MPTS) having a data rate of 27 Mb/s, a band assigned for the broadcast/video data has a data rate about 432 Mb/s even if a guard band is excluded from a consideration. The data rate of 432 Mb/s corresponds to 50% of the available bands of GbE. Accordingly, considerable waste of bandwidth occurs if the broadcast/video time-slot is not used even if a user does watch or listen to the broadcast/video.
FIGS. 2A, 2B show and OLT and ONT, respectively, using time division multiplexing in a conventional Ethernet PON structure. The conventional Ethernet PON includes, as shown in FIGS. 2A and 2B, one OLT, an optical splitter 216, and multiple ONTs, each of the ONTs being assigned, for example, to a single user
OLT 300, referring to FIG. 2A, includes a broadcast/video channel selection switch 21, a broadcast/video time-slot multiplexer 22, a broadcast/video channel selection control part 23, an IP router 24, and Ethernet PON OLT function processing part 25, a scrambler controller 26, a frame multiplexer 27, and Ethernet time-slot matching buffer 28, and electro-optical converter 209 and an opto-electrical converter 210. The switch 21 performs switching for MPEG (Motion Picture Experts Group) broadcasting and video data. After receiving selection channel information from ONTs 200-1 to 200-16, the broadcast/video channel selection control part 23 delivers, to the broadcast/video channel selection switch 21, control signals for selecting broadcast/video channels. The broadcast/video time-slot multiplexer 22 connected to the broadcast/video channel selection switch 21 performs time division multiplexing for broadcast/video channels selected by each subscriber in one time-slot. IP router 27 is used for routing communication data to an upper layer IP network or an Ethernet PON OLT function processing part 25 for processing Ethernet-PON OLT functions. The Ethernet time-slot matching buffer 28 stores communication data from the Ethernet PON OLT function processing part 25 to be sent to an ONT. The communication data is matched with broadcast/video signals time-division multiplexed so as to deliver matched data to the ONT. Frame multiplexer 27 multiplexes into one frame broadcasting/image signals of the broadcast/video time-slot multiplexer 22 and Ethernet communication signals of the Ethernet time-slot matching buffer 28. The optical transmitter 209 optically modulates frame multiplexed signals for subsequent transfer of the modulated frame multiplexed signals λdown. The optical receiver 210 receives upstream optical signals from the ONT and converts them into electrical signals. WDM coupler 211 performs combination/division by transmission/reception wavelength.
The ONT, referring to FIG. 2B, includes a WDM coupler 217, an electro-optical converter 218, an opto-electrical converter 219, a frame & time-slot demultiplexer 220, and E-PON ONT function processing part 221 and a broadcast/video matching unit 222. The WDM coupler 217 combines/splits wavelengths to be transmitted and wavelengths being received. The optical receiver 219 receives through the WDM coupler 217 optical signals λdown from the OLT and opto-electrically converts the signals. Optical transmitter 218 transmits data upstream to the OLT. The frame and time-slot demultiplexer 220 receives broadcast/video signals and Ethernet communication signals which have been time-slot multiplexed into respective frames, and separates the broadcast/video signals from the Ethernet communication signals. The Ethernet PON ONT function processing part 221 processes an ONT function, and the broadcast/video matching unit 222 recovers original signals from separated broadcast/video signals.
FIG. 3 illustrates the above-mentioned frame format for a single frame 31 and time-slots 32-1 to 32-n associated with that frame, for broadcast/video signals and Ethernet communication signals associated with the respective time slots. This format is employed in the Ethernet PON structure shown in FIGS. 2A, 2B.
As shown in FIG. 3, the time-slots include broadcast/video sub time-slots 33-1, 34-1, and 35-1 and Ethernet sub time-slots 33-2, 34-2, and 35-2 whose content varies according to selections made by the subscribers. In particular, the broadcast/video sub time-slot within the ith time-slot includes only broadcast/video signals, if any, selected by the ith ONT. Therefore, for example, if the broadcast/video signals have not been selected by the user for the ith ONT, the broadcast/video sub time-slot within the predetermined ith time-slot is vacant or includes null data. The data rate of the broadcast/video signals is 1.25 G/2k [b/s] (k=0, 1, 2, . . . ), for example, the Ethernet communication signals being 1.25 GbE.
Each of Ethernet sub time-slots within all time-slots may, by contrast, include communication data of any of the ONTs. For example, although the broadcast/video sub time-slot 33-1 of the first time-slot 32-1 is limited to only broadcast/video signals, if any, selected by a first ONT, the Ethernet sub time-slot 33-2 within the first time-slot 32-1 can be assigned to the Ethernet communication signaling of any of the ONTs. The same applies to other time-slots 32-2, 32-3.
However, in the conventional Ethernet PON shown in FIGS. 2A, 2B, a broadcast/video time-slot is specifically assigned to every subscriber as shown in FIG. 3 and is used only for transmitting broadcast/video. Also, when a user does not watch or listen to broadcast/video, the time-slot assigned to the user is not used. For example, if the E-PON has a 1×16 structure and broadcast/video data are carried on an MPEG transport stream (MPTS) having a data rate of 27 Mb/s, a band assigned for broadcast/video has a data rate about 432 Mb/s even if a guard band is excluded from a consideration. Herein, the data rate of 432 Mb/s corresponds to 50% of the available bands of GbE. Accordingly, if the broadcast/video time-slot is not used even if a user does watch or listen to the broadcast/video, serious waste of bandwidth incurs.